CARDIAC-LOOP FORMATION IN CHICK 381 
(pl. 3, C, D), and is rapidly obliterated except at the caudal end 
of the heart (pl. 3, E). Thus the heart tube, being attached only 
at its two ends, is more free to undergo extensive and rapid 
changes in shape and position. 
Even before the bending of the heart to the right has reached 
its maximum, torsion of the embryo’s body begins to change 
the mechanical limitations in the cardiac region. As the ceph- 
alic part of the embryo comes to lie on the yolk on its left side 
(fig. 1, D, E, F) the heart, no longer closely confined between 
the body of the embryo and the yolk, begins to swing somewhat 
ventrad and lies less closely against the dorsal body wall of the 
embryo (pl. 2; ef. C and D with E and F). 
The initiation of torsion has another very definite influence on 
the heart. Since torsion involves the cephalic region of the 
embryo first and progresses caudad, the body of the embryo 
becomes more inclined toward the yolk at the level of the ceph- 
alic attachment of the heart than at the level of its caudal 
attachment. As a result, the truncus arteriosus is twisted by 
the carrying of its attached end away from the yolk before a 
similar twisting effect is exerted upon the sino-atrial region of 
the heart (fig. 1, E, F, G). This is, I believe, the primary me- 
chanical factor in starting the transformation of the U-shaped 
bend into the cardiac loop. Once the initial twist is imparted, 
loop formation progresses extremely rapidly, for the rate at 
which the heart tube is outgrowing the pericardial chamber in 
length is exaggerated at this time. ‘The distance between the 
attached cephalic and caudal ends of the heart is actually being 
shortened by the progress of flexion in the embryo at just the 
time when the heart tube is elongating most rapidly (fig. 1, 
G, H, I, and fig. 2). The attached truncus and sinus ends of the 
heart are thus brought closer together, tightening the loop as it 
is formed. In the formation of the loop the truncus and bulbus 
swing away from the yolk (i.e., toward the embryo’s right), 
and come to lie across the caudal part of the heart, at the etrio- 
ventricular constriction (pl. 2, G, H, I). The sino-atrial region 
being anchored to the body walls by the remaining part of the 
dorsal mesocardium, the ducts of Cuvier, and the omphalomesen- 
teric veins, undergoes little change in position. 
